roughskinjrz

i got my 489 stroker kit in the mail. looking over everything turns out everything looks good EXCEPT wrong pistons. they turn out to be a -17cc Dome Keith Black KB 398 piston. i ordered the KB 284 Flat Top +3cc. now with the KB398's nd 119c head i get 9.9:1 compression, with the KB284's i get like 8.6:1 or something with a 119cc head.

now my question is, since i dont want to return these, they will cover shipping, but more duty fees, rebalancing with rods, etc. is 9.9:1 compression OK to run with 5lbs of boost with a roots blower??

Apeiron

Revenue Canada will reimburse your HST for a returned item.

F-BIRD'88

To run any kind of reliable boost that is worth while, using pump gas (92 octane) without a inter cooler or water/methanol injection, you want a low-low compression ratio.
9.9:1 is way too high. You ideally want 7.5:1. Allows 10-12lbs boost on 92 octane. This combination gives maximum reliable power and street performance. You can get by and do pretty good with a 8.5:1 cr, some spark retard and 6-8psi boost.

A big bore motor is much more detonation sensitive than a small bore motor is.
excessive compression ratio will just bite you in the ***.
you definatly do not want a piston with a dome. Get a dished piston or a flat top at least. This piston has a solid dome. you can likely mill the whole dome off. The piston will be lighter. the balance will be different.
The minor cost of returning the pistons (or the whole kit) and getting the right pistons/kit for what you are trying to do there is nothing compared to building the motor twice cause ya ****ed up.

BBC with a roots blower without intercooling max reliable performance on pump gas calls for a 7.5:1 cr.

A hyper piston can be reliable in a blown application but only if detonation is strickly avoided. Do not build this motor using a hyper piston and excessive cr.
You will not save money.

You should build this motor using a KB287 454 piston and stock length rod and 4" crank. Your finished cr on a 0 decked block would be 7.42:1 (+.030piston 119cc head)
You would be able to run 12lbs boost reliably all the time on pump gas with little or no spark retard and full reliability. It will make more power every day and last longer than a 489ci motor with excessive cr and very very limited boost.

You could even go .100" overbore size with a 7.6:1 finished cr and get a fully reliable 475ci 12psi boost blower motor that runs all day on pump gas (92) and makes max street pump gas power. 454+ .100" based motor.

Make the ring end gaps biggie sized. Bigger is better.

You need to rethink the combination.
If you want the most reliable supercharged power, do what the Marine pro engine builders (like Jimmy Oddy etc) do. Let the blower make the power for you. Let the blower do what it does best. Make boost. Run a high(er) boost on the blower with a low(er) engine cr and little or no power robbing, exhaust valve eating spark retard.
All that work, custom parts and money for just 5psi of boost. What a waste.

roughskinjrz

ok i asked this question along time ago and noone answered so i asked around and this is what i came up with.

anyway i got a hyper-p piston 398, i will have to send it back for the 284 whcih gives me 8.6:1 compression. i am running 6 pounds at most like i stated not the 12 you said. i appreciate your help, so ill send the pistons back i do not want to blow the motor up.

roughskinjrz

ok if it makes a difference i guess i have the 174 version. i looked up the part number on the crank pulley 90830 and it says its for 174. i cant see any numbers on the blower cuz it was polished. and i cant find where there was even a 142 for a big block.